(Supported in part by NIH BM 30758 to G. Sluder and NIH R01 GM40198 to C. Rieder). It has been hypothesized that animal cells delay anaphase onset, and thus exit from mitosis, when the mitotic spindle lacks a normal bipolar symmetry. To evaluate this hypothesis we used the BMIRR video-LM facility to film multinucleated PtK1 cells that formed multipolar spindles. An analysis of the data revealed that anaphase onset is delayed in monopolar spindles containing unattached kinetochores, but not in multipolar spindles which do not contain unattached kinetochores. This data reveals that the metaphase-anaphase Cellular cycle checkpoint does not monitor spindle geometry. It further suggests that monopolar spindles are check pointed not because they lack the proper spindle geometry, but because they have unattached kinetochores that signal wait anaphase. Sluder, G., E.A. Thompson, F.J. Miller, J. Hayes and C.L. Rieder. 1997. The checkpoint control for anaphase onset does not monitor excess numbers of spindle poles or bipolar spindle symmetry. J. Cell Sci., 110:421-429.